This is an application for an extension of a MERIT award to study natural killer T cells (NKT cells). Evidence of progress during the previous funding period is supported by 30 publications. Some of the main published findings were; 1) Determination that NKT cells can be activated independently of antigen presentation, particularly in the presence of IL-12. This mechanism allows NKT cells to become activated during viral Infections. 2) The identification of antigens from pathogenic Borrelia burgdorferi spirochetes that can activate NKT cells. These glycolipid antigens have diacylglycerol as the lipid moiety, a lipid that is found in many pathogenic microbes. We showed that NKT cells are important for preventing arthritis in a mouse model of Lyme disease. 3) We have described a mechanism for the THI skewiing of NKT cell cytokine responses that is not dependent on TCR affinity, but which is dependent on increased antigenic stability in vivo. This finding may aid in the development of new ligands for NKT cell activation. In the extension period, we will invesitgate the transcriptional control of Thi and Th2 cytokine synthesis by NKT cells. We also will analyze how NKT cells contribute to host defense in the lung following infection with S. pneumoniae. Additionally, we will explore how components of the autophagy machinery influence NKT cell differentiation and CD1d antigen presentation.